Using an electric mixer on high speed, beat egg whites until frothy. With mixer running, gradually beat in ½ cup sugar; beat until firm peaks form.

Gently fold egg whites into chocolate mixture in 2 additions, folding just until incorporated between additions. Scrape batter into prepared pan; smooth top and sprinkle with remaining 2 Tbsp. sugar.

Bake until top is puffed and starting to crack and cake is pulling away from edge of pan, 35-45 minutes. Transfer to a wire rack and let cake cool completely in pan (cake will collapse in the center and crack further as it cools).

DO AHEAD: Cake can be made 1 day ahead. Cover in pan and store airtight at room temperature.

Topping

Using an electric mixer on medium-high speed, beat cream, mascarpone, and powdered sugar in a medium bowl until soft peaks form.